Outer conductor for coaxial cable

ABSTRACT

The outer conductor of a coaxial cable is formed from a transversely corrugated metal strip comprising a laminate of steel and copper. The strip is applied longitudinally and folded about the cable with its edges in butting relation. A narrow transversely corrugated steel tape is placed over the abutting edges of the laminated strip and soldered thereto throughout the length of the cable.

United States Patent [72] Inventor Rama lyengar 3,183,300 5/1965Jachimowicz et a1 174/106 Dorval, Quebec, Canada 2,589,700 3/1952.lohnstone 174/ 106 [21] Appl. No. 45,980 3,261,906 7/1966 Gee-Wah etal. 174/107 [22] Filed June 15, 1970 3,087,007 4/1963 Jachimowicz...174/110 [45] Patented Jan. 11, 1972 2,697,772 12/1954 Kinghorn 174/102[73] Assignee Northern Electric Company Limited 3,504,102 3/1970Polizzano 174/107 Montreal, Quebec, Canada 3,070,274 12/1962 Elam138/156 UX FORElGN PATENTS Germany 7 Claims, 3 Drawing Figs. PrimaryExaminer-Thomas J. Kozma 52 u.s.c| 174/106 I),

174/28, 174/36, 174/102 D, 174/107 Ammey" PhlhP Erlckm" [51] Int.Cl H0lb7/18 [50] Field of :2 ABSTRACT: The outer conductor of a coaxial cableis formed 151 from a transversely corrugated metal strip comprising alaminate of steel and copper. The strip is applied longitu- [56]References Cited dinally and folded about the cable with its edges inbutting relation. A narrow transversely corrugated steel tape is placedUNITED STATES PATENTS over the abutting edges of the laminated strip andsoldered 3,586,754 6/1971 Meskell, Jr. 174/106 thereto throughout the|ength f the Cab|e PATENTEU JAN I 1 I972 SHEET 1 BF 2 //v VENTOR} Y RIYENGAR AGENT FIG. 3

\4 INVENTOR R. IYENGAR 55 AGENT OUTER CONDUCTOR FOR COAXIAL CABLE Thisinvention relates to an outer conductor for coaxial cable.

A coaxial cable comprises a conductor centrally spaced within a tubularouter conductor by means of a suitable dielectric. The most suitabledielectric is air which has a low specific inductive capacitance and alow dissipation factor. Therefore, it is usual to space the outerconductor coaxially about the inner conductor by means of thininsulating discs at spaced intervals.

Suitable conductive materials for the outer conductor are veryexpensive, and for this reason, it is desirable to use as thin amaterial as possible and only as thick as is electrically necessary.When this relatively weak outer conductor is supported at intervals fromwithin it is susceptable to being crushed when subjected to radial loadsor bending stresses, and this will distort the electricalcharacteristics of the cable.

In disc-insulated coaxial cables, the outer conductor has consisted of alongitudinal copper tape having serrated edges abutting and interlockingto form a seam. Strength requirements have been obtained by theapplication of helical steel ribbons over the outer conductor.

A recent development, disclosed in an article in the Bell LaboratoriesRecord of July 1969, describes an outer conductor formed from alaminated strip of copper and steel. The steel layer is wider to oneside than the copper layer so that when the strip is appliedlongitudinally and formed about the cable, with the edges of the copperlayer in the strip in butting relation, the projecting portion of thesteel layer along the edge overlaps the other edge of the strip and issoldered to it throughout the length of the cable.

This design, however, has several disadvantages. The design of theoverlap causes a ridge to be formed on the inner surface of the seamwhich results in distortion of electrical characteristics of the cable.Furthermore, the overlapping portion of the steel layer must bespecially formed to allow intimate contact with the underlying portionof the outer conductor to permit proper soldering. Yet anotherdisadvantage is the difficulty of producing the laminated strip with oneof the laminae being wider than the other.

In accordance with the present invention, the outer conductor of acoaxial cable comprises a laminated strip of two dissimilar metalsapplied longitudinally to the cable and formed into a tube with theedges of the strip in butting relation to one another to form a seam. Anarrow metal tape is applied over the mating edges of the strip and issoldered thereto on either side of the seam. The metal of the laminatedstrip on the inside of he tubular outer conductor is highly conductiveand forms a layer substantially thinner than the outer metal layer ofthe laminate. The outer metal layer and the metal tape overlying theseam are of a metal or metals having substantially greater resistance tomechanical stress than the metal of the inner layer. In a preferredform, the invention contemplates an outer conductor of a coaxial cableformed from a transversely corrugated laminated strip of copper andsteel, with the tape over the seam being of steel and having transversecorrugations nesting snugly in the corrugations of the underlying outerconductor.

A reinforced tubular conductor formed in accordance with this inventionis perfectly circular and has no ridges on its inner surface to affectthe electrical characteristics of the cable. Where the conductor iscorrugated, the tape covering the seam can be formed with complementarycorrugations so that it will fit closely to the conductor. Theintroduction of the solder ribbon is made simpler since it is applied inline with the overlying metal tape; it need not be wedged between theoverlapping edges as is required with the overlap design. If this cableis stranded with others to form a multiunit cable, shearing stressesoccuring at the soldered seam will not be borne by the soldered joint aswould be the case with an overlap design. Should the solder creep in theconventional overlap configuration, the overlapping edges would slideone over the other causing dimensional deformation of the outerconductor. The

cost of producing the laminated strip in the present invention is keptto a minimum since both laminae are exactly the same width.

The invention will now be described with reference to a preferredembodiment thereof as shown in the accompanying drawings in which:

FIG. 1 is a perspective view of a coaxial cable having an outerconductor in accordance with this invention;

FIG. 2 is a partial transverse section of the coaxial cable in FIG. 1illustrating the seam construction of the invention; and

FIG. 3 is a schematic diagram illustrating the steps in the manufactureof the coaxial cable of FIG. 1.

FIGS. 1 and 2 illustrate the general structure of a coaxial cable inaccordance with the present invention. In FIG. I, a number of dielectricdiscs 10 are disposed at spaced intervals upon a center conductor 12 tocoaxially support a transversely corrugated outer conductor 14 to formthe coaxial-cable 16. The discs 10 are slipped transversely onto thecenter conductor 12 by means of slot 18 and tightly fitting centeropening 20. Where required, a thermoplastic jacket 22 may be extrudedover the coaxial cable 16. For additional protection, a

thermoplastic flooding compound 24'may be applied over the cable 16prior to the application of jacket 22.

As shown more clearly in FIG. 2, the outer conductor 14 is formed from astrip of laminated metal. The laminate comprises a steel strip 26,having layers of tin 28 plated on its opposite faces, bonded by means ofa plastic adhesive bonding strip 30 to a copper strip 32 of the samewidth. The outer conductor 14 is formed from the transversely corrugatedlaminate by forming it about and in close contact with the spaceddielectric discs 10 with the edges 34 of the laminate in buttingcontact.

A narrow steel tape or buttstrap" 36, having layers of tin 38 plated onits opposite faces, and having a pluralitY of adjacent transversecorrugations 40 (FIG. 1), is centered over the seam 42 formed from theabutting edges 34 of the outer conductor 14. Corrugations 40 arecomplementary to the corrugations of the outer conductor 14 to ensureintimate contact between the buttstrap 36 and the outer conductor. Asolder joint 44 is formed between the outer conductor and the buttstrap36 as shown in FIG. 2.

The cable thus formed has an outer conductor comprising a small quantityof relatively expensive and conductive copper and a larger quantity ofrelatively inexpensive but much stronger steel. The total thickness isconsiderably less than would be required for an all copper outerconductor having comparable structural qualities. The tin plating makesthe layer of steel 26 and the buttstrap 36 solderable at relatively highspeeds while employing very small quantities of soldering flux.

The diameter of the coaxial cable 16 is approximately 0.430 inch overthe outer conductor 14. The outer conductor is formed from a laminate 1%inches wide. The copper strip is 4 mils thick, the steel strip is 10mils thick and the adhesive between them is 2 mils thick. The adhesiveis preferably a thermosetting polyester in strip form. The steel buttstrap 36 is one-fourth inch wide and 6 mils thick. The jacket ispreferably polyethylene about 50 mils thick.

The coaxial cable of FIGS. 1 and 2 is manufactured as shown in theschematic diagram of FIG. 3.

In the process illustrated in FIG. 3, the components of the coaxialcable 16 are drawn from their various supplies and through theprocessing steps by capstan 46 which draws the completed cable forwinding on takeup reel 47. The center conductor 12 is drawn from give-upreel 48 and passes through disc applicator 50 which puts the disc-shapeddielectric spacers 10 on the conductor 12 at intervals along its length.A suitable disc applicator 50 is disclosed in Canadian Pat. No. 494,928,issued on July 28, 1953 to A. C. Frankwich.

A tin-coated steel strip 26 is drawn from a roll 52 and a copper strip32 is drawn from roll 54. Both strips pass through a degreaser unit 56then through heaters 58 and 60. The heated strips are brought togetherat laminating rolls 62 with an adhesive strip 30, drawn from supply roll64, between them. The steel strip 26 and the copper strip 32 carrysufficient heat from heaters 58 and 60 to soften the adhesive 30 andcauses the steel 26 and copper 32 to bond together into a laminatedstrip when the three layers are pressed together by laminating rolls 62.The resulting laminated strip is air-cooled and passes through a set ofcorrugating rolls 66 which corrugates the laminate transversely to itslongitudinal direction. To ensure proper tension in the corrugatedstrip, the laminate is drawn through a tension control loop 68.

Both the center conductor 12, carrying the discs 10, and the corrugatedlaminate converge at a suitable roll former 70. The roll former curvesthe laminated strip transversely and forms it into a tube about thediscs with the longitudinal edges of the laminated strip in buttingrelation. The resulting tube which forms the outer conductor 14 is thenpassed through a set of guide rolls 72 which prevent it from rotatingabout the axis of the cable and keep the seam in proper alignment forthe application of the buttstrap 36.

The buttstrap 36 is drawn from roll 74 and passes through a set ofcorrugating rolls 76 which forms transverse corrugations of suchdimensions that the resulting corrugations will register with thecorrugations of the outer conductor 14. The tension of the buttstrap 36is controlled by a tension control loop 78.

A solder wire is simultaneously drawn from a coil 80 and passes througha set of flattening rolls 82, which rolls it into a flat solder ribbon84 having the same width as the buttstrap 36. A suitable eutectic solderwire comprises 63 percent tin and 37 percent lead. Prior to rolling thesolder wire is approximately 55 mils in diameter; it is rolled to form aribbon onequarter inch wide and 3 mils thick. The solder ribbon 84passes through a flux bath 86 and converges on the outer conductor 14under the buttstrap 36 at a set of registering rolls 88. These rollsensure that the corrugations of the buttstrap 36 register properly withthe corrugations of the outer conductor 14 and also curve the buttstraptransversely to the contour of the outer conductor so that the buttstrapand outer conductor mate snugly together. The cable is advanced to asoldering station 90 where heat is applied in the area of the seam inthe outer conductor to melt the solder and fuse the buttstrap 36 to theouter conductor 14. Cable 16 passes through a cooling device 92 and isfed by the capstan 46 onto takeup reel 47. A flooding compound 24 and aprotective jacket 22 may subsequently be applied if required.

This process has been found to operate satisfactorily at speeds up to150 feet per minute.

The process described hereinabove produces a coaxial cable having a muchgreater crush resistance than a disc-insulated coaxial cable havinghelical steel tapes applied over a thin copper outer conductor. ltsbending qualities are such that is may be flexed over much smaller radiiwithout significant deformation of the outer conductor. The solderedbuttstrap design provides a smooth inner surface of the outer conductorat the seam and affords an effective seal against the ingress ofmoisture. Because the conductive portion of the outer conductor iscompletely covered by a steel layer which is sealed at the seam bysoldering, radiation is effectively eliminated. Thus multicoaxial cableusing adjacent coaxials suffer very little from crosstalk.

it will be obvious to those skilled in the art that aluminum can besubstituted for the conductive copper layer without any deterioration ofthe above-mentioned characteristics. In that case, the aluminum wouldpreferably be 6 mils thick.

Where sealing ofthe seam in the metal layer is not essential, thesoldering or other joining technique need not be continuous throughoutthe length of the cable, subject to strength requirements of theconstruction. This may be of practical application where the metal layerand the buttstrap are coated with a suitable bonding material or othersynthetic material.

What is claimed is:

1. An outer conductor for a coaxial cable comprising:

' a metal strip folded transversely about an insulated center conductorto form a tube with theed es of the strip extending longitudinally ofthe cable in a uttmg relation to form a seam, said strip being alaminate of two layers of dissimilar metals, the one of said two layerson the inside of said tube being of a highly conductive metal and theother of said layers being substantially greater in thickness than saidone layer and being of a metal having substantially greater resistanceto mechanical stress than the metal of said one layer; and

a metal tape extending longitudinally of and overlying said seam andbeing soldered to the metal strip throughout its length on either sideof the seam to seal the seam and prevent the edges of the strip frombeing displaced from the abutting position, said tape being of a metalhaving substantially greater resistance to mechanical stress than themetal of said one layer.

2. An outer conductor for a coaxial cable as defined in claim 1 whereinthe metal tape is substantially thicker than said one layer.

3. An outer conductor for a coaxial cable as defined in claim 2 whereinthe metal strip and the metal tape have transverse corrugations, thecorrugations of the metal tape being complimentary to the corrugationsof the metal strip to permit intimate engagement ofthe metal tape withthe metal strip.

4. An outer conductor for a coaxial cable as defined in claim 3 whereinthe metal of said other layer is steel.

5. An outer conductor for a coaxial cable as defined in claim 4 whereinthe metal of said tape is steel.

6. An outer conductor for a coaxial cable as defined in claim 5 whereinthe metal of said one layer is copper.

7. An outer conductor for a coaxial cable as defined in claim 6 furthercomprising a polymeric material disposed between and bonding togetherthe two layers of the laminate.

1. An outer conductor for a coaxial cable comprising: a metal stripfolded transversely about an insulated center conductor to form a tubewith the edges of the strip extending longitudinally of the cable inabutting relation to form a seam, said strip being a laminate of twolayers of dissimilar metals, the one of said two layers on the inside ofsaid tube being of a highly conductive metal and the other of saidlayers being substantially greater in thickness than said one layer andbeing of a metal having substantially greater resistance to mechanicalstress than the metal of said one layer; and a metal tape extendinglongitudinally of and overlying said seam and being soldered to themetal strip throughout its length on either side of the seam to seal theseam and prevent the edges of the strip from being displaced from theabutting position, said tape being of a metal having substantiallygreater resistance to mechanical stress than the metal of said onelayer.
 2. An outer conductor for a coaxial cable as defined in claim 1wherein the metal tape is substantially thicker than said one layer. 3.An outer conductor for a coaxial cable as defined in claim 2 wherein themetal strip and the metal tape have transverse corrugations, thecorrugations of the metal tape being complimentary to the corrugationsof the metal strip to permit intimate engagement of the metal tape withthe metal strip.
 4. An outer conductor for a coaxial cable as defined inclaim 3 wherein the metal of said other layer is steel.
 5. An outerconductor for a coaxial cable as defined in claim 4 wherein the metal ofsaid tape is steel.
 6. An outer conductor for a coaxial cable as definedin claim 5 wherein the metal of said one layer is copper.
 7. An outerconductor for a coaxial cable as defined in claim 6 further comprising apolymeric material disposed between and bonding together the two layersof the laminate.